1. Field of the Invention
The present invention relates to a flat type optical cable which is integrated by coating a plastic coating layer on parallel arranged optical-fiber cords, particularly relates to a flat type optical cable which is easily subjected to terminal treatment.
2. Description of the Related Art
A flat type optical cable having such a cross-sectional structure as shown in FIG. 2 is heretofore mainly used for indoor wiring. The revealing work for taking optical-fiber cords out of the optical cable by removing a part of an outer coating (hereinafter referred to as cable jacket) at an end portion of the optical cable is required when a connector for connecting the optical cable to an apparatus is to be attached to a terminal of the optical cable or when the optical cable is to be connected to another optical cable.
Specifically in this revealing work, first, the flat type optical cable is cut longitudinally by about 10 mm by means of a nipper from above the cable jacket along the respective optical-fiber cords in the inside of the flat type optical cable at the end portion thereof. The thus cut pieces of the cable jacket are picked by fingers and pulled in mutually opposite directions so as to be torn by a predetermined length. Thus, the respective optical-fiber cords in the inside of the optical cable are exposed.
Incidentally, after revealing has been performed for exposing the optical-fiber cords at the end portion, the cut portions of the cable jacket are removed. Then, a connector is attached to the optical cable so that the optical cable is connected to an optical apparatus, another optical cable, or the like.
FIGS. 1 and 3 show examples of the state in which revealing was performed on an end portion of a flat type optical cable. FIG. 1 shows a desirable example in which respective optical fiber cords could be taken out without any injury of the cord jacket. FIG. 3 shows an undesirable example in which respective optical fibers in the inside of the optical-fiber cords were exposed because the cord jackets as well as the cable jacket were torn out.
Incidentally, a flat type optical cable is generally constituted by from one to about five optical fiber cords. A flat type optical cable is mainly used indoors and shaped like a flat plate in cross-sectional view in order to facilitate the work of laying or wiring along wall or floor surfaces.
Each of the optical-fiber cords contained in the flat type optical cable has an optical-fiber core or a bare optical fiber 1, an aramid fibrous layer 2 (for example, trade name: Kevlar) longitudinally attached onto the optical-fiber core or the bare optical fiber 1, and an outermost layer 3 of plastics such as polyethylene or polyvinyl chloride with which the fibrous layer 2 is coated.
JP-A-8-262284 shows the general technical level of such a flat type optical cable. This publication discloses that the toughness of the optical-fiber cords can be secured while the flexibility of the optical cable as a whole can be secured when the hardness of the cord jackets is set to be in a range of from 40 degrees to 60 degrees in terms of Shore durometer hardness and the hardness of the cable jacket is set to be in a range of from 20 degrees to 35 degrees in terms of Shore durometer hardness.
In the background art, however, as shown in FIG. 3, when the flat type optical cable is subjected to revealing, there is a possibility that the optical-fiber cords may be damaged because the outer coating (hereinafter referred to as cord jacket) of each of the optical-fiber cords as well as the cable jacket are torn out.
If there is such a damage, the revealing work is required to be tried again. As a result, the efficiency of the work is lowered, so that the laying cost increases as well as the laying term is elongated.
It is an object of the present invention to provide a flat type optical cable in which it is possible to take optical-fiber cords out at an end portion of a flat type optical cable without any damage of the optical-fiber cords to thereby improve efficiency in the work of treating a terminal of the flat type optical cable.
The present invention provides a flat type optical cable comprising a plurality of optical fiber cords, each including an optical fiber coated with a cord jacket, which are parallel arranged in a plane, and a cable jacket which directly extrusion-coats the plurality of optical fiber cords so that the optical fiber cords are integrated; wherein the Young""s modulus of a coating material forming the cable jacket is lower than the Young""s modulus of a coating material forming the cord jacket.
This is because, if the strength characteristics of the cord jackets and the cable jacket are selected as described above, the cord jackets are high in Young""s modulus and hardly expand so that the cable jacket is easily peeled and separated from the cord jackets when the cable jacket at an end portion of the optical cable is pulled for the revealing work.
Even if there is a region in which the cable jacket is partially fusion-bonded to the cord jackets, the cord jackets of the optical-fiber cords are prevented from being deformed or damaged because the cable jacket in this region expands more greatly than the cord jackets so that the cable jacket is broken and separated from the cord jackets.
The present invention is particularly adapted to the flat type optical cable in which the Young""s modulus of the coating material forming the cord jacket is in a range of from 9 kg/mm2 to 15 kg/mm2 whereas the Young""s modulus of the coating material forming the cable jacket is in a range of from 4 kg/mm2 to 9 kg/mm2.
Further, the present invention provides the flat type optical cable in which the extrusion coating temperature for the cable jacket is lower by at least 10xc2x0 C. than the extrusion coating temperature for the cord jacket.
This is because, if a combination of materials having the aforementioned relation is selected in terms of extrusion coating temperature characteristics of plastic materials for the cord jackets and the cable jacket, the cable jacket can be formed by extrusion coating at a lower temperature than the cord jackets and fusing of the cord jackets with the cable jacket hardly occurs when the cable jacket is formed on the cord jackets of the optical-fiber cords by extrusion coating.
Incidentally, the extrusion temperature can be originally selected to be in a considerably wide temperature range. In the present invention, however, a temperature obtained by a melt-flow test which will be described later as a test for specifying an optimum extrusion temperature for each of plastic materials of the cord jackets and the cable jacket is defined as the extrusion temperature, so that extrusion coating is performed at this temperature.
Further, the present invention provides the flat type optical cable in which: the Shore durometer hardness of the cord jacket is higher than that of the cable jacket; and the Shore durometer hardness of the cable jacket is in a range of from 40 degrees to 60 degrees.
Hardness and surface smoothness are substantially different concepts from each other. Generally in the case of a plastic material, the extrusion-molded surface becomes denser and smoother in terms of surface roughness as the hardness becomes higher. Accordingly, when a plastic material high in hardness is selected as the coating material for the cord jackets, the surfaces of the cord jackets are smoothed. Hence, even if there is an unevenness in the surfaces of the cord jackets when the cable jacket is formed on the cord jackets by extrusion coating, each recess in the unevenness is so small that a molten resin hardly enters the recess. As a result, the cable jacket is hardly bonded to the cord jackets, so that the cable jacket is easily separated from the cord jackets when revealing is performed.